In addition to having a physical body, people (and every other living creature) also have an energy body. This energy body is called the aura. It has seven layers and is situated mostly outside the body. The first layer, the energy double, is situated partly in the physical body, the other layers are outside it.
The layers closest to the body are the energy double, the emotional layer and the mental layer. Each layer has its own function. The energy double is connected closely to the physical body and is an exact copy of it. In the emotional layer all your emotions are stored, just as all thought patterns are stored in the mental layer.
Your aura connects you to the auras of other people, and helps you to sense energies. If your aura is damaged, or you feel out of sorts, you will lose energy and have trouble getting rid of the energies that stick to your aura. You can probably imagine situations in which you feel tired and dirty after a day in the company of other people, even though you haven't done anything but sit in a room all day.
Some people are very skilled at draining energy from others, like people who need lots of attention and constantly claim you. Sometimes you give away your energy yourself, maybe because you are less grounded or subconsciously believe someone else has more rights to it than you. Maybe you can't bear to see someone suffer and give them your energy to lighten their suffering. This sort of thing happens very subtly and it is very hard to pinpoint afterwards what it was that has drained all your energy. All this can lead to you feeling tired, possibly resulting in physical and mental discomfort.
An aura is visible, though some people will see it faster and see more than others. You'll probably recognize this: you're just sitting, staring a bit and suddenly you see a kind of grayish white colour around a person or a tree, but when you wanted to look at it more closely it was gone. Well, that was an aura and with some practice you can look longer and often see colours as well. It's a certain way of looking, not quite staring into nothing, but something just short of it. I read somewhere that you should look with your peripheral vision. Once when we were on the way home from a very tiring day I saw this beautiful deep pink colour above the trees. That is, all the trees except above the one I was looking directly at, so finding the bit about the peripheral vision made a lot of sense to me.
The aura is very colourful and those colours can change from second to second, depending on your physical, mental and emotional state. For instance, yellow can mean you are very intellectually orientated, or that you are thinking about something right then. Light blue can appear when you are enthousiastically talking about your favourite hobby and when you're angry dark red is coming off you in waves and spikes, but if you are very sad the red of that emotion will stay close to you instead.
In the aura you will find the chakras. They are energy wheels distributing the energy you take in from the earth all over your energy system. Blocks in these wheels can also lead to discomfort, because the energy will stagnize and be distributed less efficiently. On the chakra page there's more on chakras.
There is so much more information on this subject that I can give you here, so if you want to know more about it, have a look on the books page for a list of books.